Probing involves contacting a pad surface of an integrated circuit with a probe tip. The process involves positioning of probe pads relative to probe tips. The positioning of probe pads, in one system, is achieved by moving the wafer containing the devices under test. From a set of pads under test to the next set of pads, the motion consists of moving the pads away from the pins, moving the wafer such that the next set of pads are under the probe tips, and moving the pads toward the probe tips to make contact with the probe tips.
Since extreme precision is involved in positioning the pads under the pins, it is necessary to control the mechanical motion of the wafer precisely. Any unwanted motion in cross directions can damage the device under test. Consequently, extreme care is taken to ensure that mechanical motions are well controlled before making contact with the probe tips.
FIG. 1a shows a common stage motion profile 20 where a stage moves from a contact position 22 to a clearance height position 24 by single axis motion (often in a vertical or “Z” direction). The clearance height is defined as the height at which the stage can be moved safely without damaging system components. The stage then moves to another commanded stage position 26, equal to the height of the clearance height, whereafter the stage moves to another contact position 28. The entire motion profile 20 is shown by three line segments 30, 32, 34.
FIG. 1b shows an acceleration profile superimposed on the segments 30, 32, 34. The first acceleration profile has two segments 36, 38 that can be viewed with reference to the acceleration axis a1 and distance axis d1. The second acceleration profile having two segments 40, 42 can be viewed with reference to the acceleration axis a2 and distance axis d2. The third acceleration profile having two segments 44, 46 can be viewed with reference to the acceleration axis a3 and distance axis d3. The segments 36, 38, 40, 42, 44, and 46 do not represent a direction of movement; rather, they show periods of acceleration or deceleration.
The first acceleration profile shows an acceleration 36 to a midpoint 48 being equidistant between the contact position 22 and clearance height position 24. After reaching the midpoint 48 distance, the stage decelerates during the second segment 38 until the stage reaches a stopping point at the clearance height 24. It will be understood that the only motion which occurs in the first acceleration profile is a vertical motion which includes both a period of acceleration (segment 36) and a period of deceleration (segment 38). The second acceleration profile shows an acceleration 40 and a deceleration 42 with a midpoint 50. It will be understood that the only motion which occurs in the second acceleration profile is a horizontal motion which includes both a period of acceleration (segment 40) and a period of deceleration (segment 42). The third acceleration profile has an acceleration 44 and a deceleration 46 to arrive at another contact position 28. It will be understood that the only motion which occurs in the third acceleration profile is a vertical motion which includes both a period of acceleration (segment 44) and a period of deceleration (segment 46).
Therefore, in the Z-direction, half of the movement in the Z-direction is spent accelerating while the other half of the movement in the Z-direction is spent decelerating.
The single axis motion described requires a verification that motion is complete and disturbances are minimized before moving the wafer in the next axis. This is necessary for both wafer and probe card safety.